It seems every CFer responds differently. For instance I HARDLY EVER get fevers, usually only when I get the flu or a cold (in other words, a 'normal person' bug).
But the simple fact is I'm in tune enough with my body that I can feel when things are 'not normal'. Usually these manifest as:
Either thicker, greener mucus (usually a sign that I'm not getting enough rest, be it sleep or work) or a complete lack of mucus (not being able to cough anything up).
Constant fatigue, I'm ALWAYS in the mood for a nap, but if I literally can't live without one.. I take note.
Sinus stuffings, soar throat (usually due to sinus problems) etc.
Normal signs like weak muscles, aches, fever (though as I said that's rare for me) etc.
Usually I diagnose my health 99% on my lungs, if they are congested, , raw, mucus is too dark, etc. That is what I usually base my health on. Bugs come and go, but CF bugs effect my lungs MORE than regular bugs.
As for when do I decide about a doctor, when I feel these symptoms coming, I usually say to myself "If by x day next week I am not feeling any better, I will contact the doctor". Then I pump myself full of water, vitamins, etc. and cut back on sugar/dairy etc. And usually... by that day, I feel much better!
Edit: please note, as I said though, every CFer is different, this is the routine I've come to. If you are feeling sick and it's unusual, when it doubt, always contact the doctor. The worst they can say is "call back if it gets worse" or "Come in just to get checked out".